Compressors and turbines of gas turbine engines such as those used in aero engines typically include a plurality of rotor and stator vane assemblies. The rotor assemblies are designed to impart work into gases passing through the compressor and extract work from the gases passing through the turbine. The stator vane assemblies help to direct the working gases entering or exiting the rotor assemblies and thereby increase efficiency of the engine.
Each rotor assembly includes a disc and a plurality of blades attached to the disc so as to extend radially outwardly therefrom. Conventionally, the blades have been attached to the disc by mechanical connections such as “fir tree” type connections where a fir tree shaped blade root is received within a complementary shaped recess in the disc. This means that the blade can be readily replaced in the event that it is damaged.
Recent developments have resulted in integrally bladed rotor assemblies or “blisks” in which the blades are formed integrally with the disc. These have the advantage of reduced weight as compared to a standard rotor assemblies and in improved aerodynamic efficiency. Such blisks are particularly applicable in the design of military aero engines.
In view of the nature of their applications, blisks are susceptible to damage that in some circumstances may require the replacement of a blade. In order to replace the blade, it must be removed by machining it off to leave a stub, and welding a new blade onto the stub by linear friction welding. This is a process whereby one part is held stationary while the other part is oscillated against it under load, the heat generated and the applied loads result in a weld, as material exudes from the edges of the joint. In the repair of blisks, the replacement blade is oscillated relative to the stationary disc, while a load is applied in the radial direction towards the disc. The blade is thereby joined to the disc.
Linear friction welding results in the consumption of material (which leaves the edges of the weld as flash) and in the recirculation of contaminants back into the weld during the process. This results in the necessity to machine off significant amounts of material around the weld. In view of this, blisks have conventionally been formed with oversized fillets at the disc and blade interface in order to allow a repair stub to be machined into the aerofoil profile should a replacement blade need to be attached through the linear friction welding process. However, this can prevent optimum design and produce unnecessary weight.